usb: gadget: atmel_usba_udc: add missing ret value check

Add missing return value check. In case of error print debug message
and return error code.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>